Process for overcoating a xerographic plate



United States Patent 3,140,174 PROCESS FOR OVERCgiTING A XEROGRAPHIC PTE Harold E. Clark, Penfield, N.Y., assignor to Xerox Corporation, acorporation of New York No Drawing. Filed June 23, 1959, Ser. No.822,188 5 Claims. (Cl. 961) The present invention relates generally toxerographic plates and to techniques for producing such plates, and moreparticularly to xerographic plates of the binder type wherein the binderlayer is provided with an overcoating.

The electrophotographic or xerographic process involves thesensitization of a xerographic plate and the exposure of the sensitizedplate to an original image to be reproduced. The exposed plate isdeveloped by the use of electrostatically attractable marking particlesto produce a visible image which is thereafter transferred from theplate to a final support, the transferred image being fixed thereon toform the final print.

A standard xerographic plate is constituted by a thin layer of seleniumon a metal backing plate, the selenium being deposited thereon in avitreous form. In the dark state, the selenium layer has an extremelyhigh resistivity, but when exposed to light the resistivity is reducedmany orders of magnitude, depending on the intensity and wavelength ofthe light. By reason of its high electrical resistance in darkness theselenium layer can be charged electrostatically, which charge isretained for a prolonged period should no light impinge thereon.However, when light strikes the xerographic plate the resistivitydecreases and the charge is discharged to the backing plate.

A more recent development in the field of xerographic plates is theso-called binder plate. It has been found that an efficientelectrophotographic plate can be obtained by spraying or otherwisecoating (as by a doctor blade, dip roll, air knife, etc.) a base platewith a photoconducting insulating composition prepared by intimatelymixing and grinding together a photoconducting insulating material, abinder of a high electrical resistance and a solvent. On evaporation ofthe solvent, the composition forms a continuous adherent layer on theplate that will hold electrostatic charges in the dark and rapidly losethem on exposure to light. A typical binder plate is constituted byfinely divided amorphous selenium particles dispersed in a thermoplasticsynthetic resin binder. A binder plate may be charged, exposed anddeveloped in the same manner as a standard xerographic plate and theimage fixed by application of heat. The development system for the platemay be that involving a magnetic brush. In this system the toner iscarried by iron filings magnetized by a permanent magnet. By merelybrushing the surface of the plate with the clump of filings clinging tothe end of the magnet, toner is removed by the electrostatic image whichis then developed.

One liimtation in'the use of binder plates is that it is not possible toeifect complete transfer of the toner image by reason of the so-calledtooth on the surface of the binder plate. The term tooth refers to thesurface condition of the photoconductor binder structure. In the binderplate the surface is relatively rough so that the fine toner particlesare mechanically trapped therein and cannot be dislodged by normalcleaning methods. While a standard selenium plate can be wiped clean oftoner, a binder plate is difiicult to so clean. A selenium platereflects light specularly, indicating a glassy surface, whereas binderplates of the type heretofore known reflect light diffusely, indicatinga rough surface. Since the toner cannot be effectively removed fromknown binder plates, the plate is not readily reusable.

In view of the foregoing it is the principal object of the presentinvention to provide an improved xerographic 3,140,174 Patented July 7,1964 plate of the binder type having a smooth surface which may be wipedclean, whereby the binder plate is reusable.

More particularly, it is an object of the invention to provide axerographic plate of the binder type having an overcoating adapted tofill in the irregularities on the surface of the binder plate to producea smooth, shiny surface from which toner can easily be removed.

Yet another object of the invention is to produce an overcoating for abinder plate having a degree of hardness and resistance to abrasion sothat repeated cleanings will not result in a scratched overcoatedsurface.

A principal object of the invention is to produce a binder plate havinga degree of hardness and resistance to abrasion as to permit repeateduse in a xerographic process coupled with electrical properties so thatthere is no loss in image quality on such repetitive use.

In general, the invention resides in the use of an overcoating for abinder-type xerographic plate which is applied to the binder plate in anon-solvent for the binder in the xerographic plate.

Previous investigators have found that the application of protectiveinsulating overcoatings to binder plates significantly reduce desirableelectrostatic contrast. See, for example, the article by Dr. E. Wainerin Photographic Engineering, vol. 3, No. 1, pages 12-22 (1952). Thereason for this adverse effect on the xerographic properties is notknown. However, it has been discovered that application of theovercoating in a non-solvent for the binder in the binder plate not onlyeliminates the loss in electrostatic contrast, but, in at least somecases, actually increases electrostatic contrast on repetitive cycling.Moreover, there has been no apparent loss of adhesion between theovercoating layer and the photosensitive layer.

One method for applying an overcoating in accordance with the instantinvention is to cure or cross-link the binder in the binder plate to acompletely or partially thermo-setting condition as by adding a suitablecatalyst or polyfunctional reactant to a thermoplastic resin binder orusing a thermo-setting resin in an intermediate stage of polymerizationfor the binder and completing the polymerization on the plate. Due tothe generally insoluble nature of such resin binders, a wide variety ofsolvents may be used for applying the overcoating resin usingconventional coating processes without the solvent attacking (i.e.,softening or partially solubilizing) the binder.

Alternatively the binder resin and overcoating resin must be so selectedthat a solvent may be chosen for the overcoating which does not attackthe binder.

In any case the binder must be electrically insulating to preventdissipation of electrostatic charge by lateral conductivity and must betranslucent to the radiation activating the photoconductive layer.

The overcoating in accordance with the invention is preferably appliedto a xerographic plate of the binder type hicluding a photoconductorconstituted by a commercial grade of zinc oxide. Supplied to the zincoxide is a binder in the form of a 60% solution of silicone resin. Thezinc oxide and binder are mixed intimately in the proportion of 1.5grams of Zinc oxide to 1 gram of binder (60% solids). One gram oftoluene is added and the mixture is placed in a ball mill and mixed forabout three hours in order to attain a thorough dispersion of thephotoconductor.

The mixture is then placed in a large tank. The backing plate to becoated is lowered into the tank so that the area thereof to be coatedlies below the surface of the mixture. The backing plate may be in theform of an aluminum sheet, of an inch in thickness. It is to beunderstood, however, that the backing may be constituted by any suitableelectrical conductor or a plate having one conductive surface.

The plate is thereupon withdrawn at a constant speed from thephotoconductor binder mixture, the speed of withdrawal determining thethickness of the coating. The mixture in the tank is maintained at roomtemperature. As soon as the plate is completely withdrawn it is placedin a horizontal plane and permitted to dry. The drying operationnormally takes a few seconds. The usual coating thickness obtained andfound satisfactory in practice is in the order of 15 to 75 micronsalthough thicknesses of from about 10 to about 200 microns have beenfound operable.

Among the overcoating materials which are suitable are the following:vinyl resins such as polyvinyl alcohol, polyvinyl chloride, polyvinylacetate, polyvinyl formal, etc.; polyester resins such as boththermoplastic and thermosetting alkyds, etc. ccllulosics such ascellulose acetate, ethyl cellulose, cellulose acetate butyrate, etc.;acrylic resins such as the polwnerized lower alkyl esters of acrylic andmethacrylic acid; and thermosetting resins such as epoxides, urethanes,etc. The best solvent for the overcoating material is that which is fastdrying, smooth coating, and produces a clear film.

The overcoating may be applied in several ways as by spraying, a diproll, air knife, doctor blade; etc. One method is to dip the binderplate in the overcoating solution and to withdraw the plate at aconstant rate, and allow it to dry. The solvent evaporates rapidly,leaving a smooth, thin insulating film over the photoconductive binderlayer. In the event a thermosetting resin is used, the polymerizationmust be completed to set the overcoating resin as by heating, or by theapplication of a suitable catalyst at room temperature conditions. Theovercoating should be from about 0.1 to microns thick.

The following examples are presented by way of illustration and not byway of limitation of the invention.

Example 1 Two binder plates were prepared as described above comprisinga layer of zinc oxide in a silicone resin obtained from General ElectricCompany under the trade name SR82. The zinc oxide was a pigment gradematerial obtained from New Jersey Zinc Company under the trade nameFlorence Green Seal No. 8. The zinc oxide-silicone layers each had athickness of about 50 microns and were coated on aluminum backings asdescribed. Half of the one plate was over coated with polyvinyl formalas follows: A 3 /2 by weight, solution of polyvinyl formal (obtainedfrom Shawinigan Products Corp. under the trademark Formvar) dissolved intrichloroethylene was poured onto the plate and allowed to evaporateleaving a coating of polyvinyl formal about one micron thick. Half ofthe other plate was coated with a water-alcohol solution (76.3% ethanoland 21.7% water) of a polyvinyl alcohol. The polyvinyl alcohol used wasobtained from E. I. du Pont de Nemours and Company under the trade nameElvanol 20-105. The resulting overcoating was about 1 micron thick. Theplates were then tested in the regular xerographic process as describedby Carlson in US. 2,297,691 involving eleven successive cycles ofcharging, exposure, developing, electrostatic transfer of the image andcleaning by swabbing the surface with a cotton swab. From the very firstimage there was virtually no electrostatic contrast, i.e., no visibleimage, on the portion of the control plate overcoated with polyvinylformal. On the portion of the control plate not overcoated withpolyvinyl formal and on the portion of the polyvinyl alcohol plate notovercoated therewith, the initial image had excellent electrostaticcontrast for the first image which gradually decreased, i.e., producedsuccessively poorer images until the eleventh plate was of very poorquality. In contrast, the portion of the plate overcoated with thepolyvinyl alcohol from the first produced sharp images which actuallyimproved in quality on successive use in the xerographic process so thatthe last plate was even better than the first and significantly betterthan the equivalent plate on the unovercoated plate. At the end of ofeleventh test cycle there was no detectable deterioration in thepolyvinyl alcohol overcoated xerographic plate.

Example 2 A binder plate was prepared by mixing together 2.5 parts ofzinc oxide (Florence Green Seal No. 8), 1 part of a styrene-alkydcopolymer obtained from American Cyanamid Co. under the trademarkCycopol S 101 and 3 parts of toluene (all parts by weight) and coatingthe mixture on an aluminum backing. After drying, the resulting coatingwas about 1 mil thick. The plate was cured at 140 F. for one-half hourand then overcoated with a urethane resin prepared by adding 10 parts ofa polyester (obtained from Mobay Chemical Co. under the trade nameMultron R-4), parts of an isocyanate (obtained from Mobay Chemical Co.under the trade name Mondur CB-75 and about 500 parts of cellosolveacetate (all parts by weight) The cured Cycopol was not affected by theCellosolve acetate solution. The resulting overcoating was about 1micron thick. The overcoated plate was stored at room temperature underhigh humidity to complete curing of the isocyanate resin. On removalfrom storage over high quality xerographic images were developed andcleaned from the plate surface without noticeable deterioration orfouling of the plate. Other plates were successfully prepared with zincoxide: Cycopol ratios of up to 4: 1, both with and without dyesensitizer (Rose Bengal), at thicknesses of 0.5 to 1 mil, and withisocyanate thicknesses from 1 to 5 microns.

While the invention has been described in connection with theovercoating of binder plates wherein the photoconductor is zinc oxide,it is to be understood that other photoconductors may be used. Forexample, the invention is also applicable to binder plates wherein thephotoconductor is formed by finely divided particles of amorphousselenium in a binder of synthetic resin, such as is disclosed in US.2,663,636, issued December 22, 1953. Other binder plates to which theinstant invention is applicable include those described, for example, inco-pending application Ser. No. 668,165, filed June 26, 1957, byMiddleton et al.; Ser. No. 697,601, filed November 20, 1957, by W. G.Van Dorn et al.; Ser. No. 697,602, filed November 20, 1957, by W. G. VanDorn et al.; and Ser. No. 672,569, filed July 18, 1957, by M. A.Insalaco. The types of photoconductive pigments, operable resins, ratiosof pigment to resin, dye sensitization, backing materials and methods ofapplying the light sensitive coatings are fully described therein.

While there has been disclosed preferred overcoatings and methods ofapplying same, it will be appreciated that many modifications andchanges may be made without departing from the essential features of theinvention, and it is intended to cover such changes and modificationswithin the scope of the appended claims. Thus, the overcoatings may beapplied from an emulsion or dispersion as well as from solution.

This application is a continuation-in-part of my copending applicationS.N. 482,896, filed on January 19, 1955, now abandoned.

I claim:

1. A method of making a smooth surface binder plate which comprises:depositing a light-sensitive photoconductive material on the surface ofan electrically conductive backing plate to form thereby aphotoconductive layer having an irregular surface, said photoconductivelayer comprising a mixture of photoconductive material and a waterinsoluble resinous binder, depositing over said irregular surface anaqueous solution of a water soluble electrically insulating material,said solution used being chemically unreactive with said photoconductivematerial, allowing said liquid carrier to evaporate from said solutionand the resinous material therein to thereby deposit in and fill theirregularities of said surface with said resinous material, continuingsaid deposition until a smooth resinous film of from about 0.1 to about5 microns is formed on said photoconductive material.

2. An electrophotographic process wherein the reusable binder plate madeby the method of claim 1 is sequentially electrically charged, exposedto a light image pattern to be reproduced and developed withelectrically attractable marking particles.

3. The method of making a smooth surface reusable electrophotographicbinder plate which comprises depositing a zinc-oxide silicone resinmaterial on and in direct physical contact with the surface of anelectrically conductive backing plate to form thereby a photoconductivelayer having an irregular surface, depositing substantially evenly andentirely over said irregular surface an aqueous solution of a watersoluble electrically insulating material, said solution used beingchemically unreactive with said photoconducting material, allowing saidliquid carrier to evaporate from said solution and the resinous materialtherein to thereby deposit in and fill the irregulan'ties of saidsurface with said resinous material, continuing said deposition until asmooth resinous film of from about 0.1 to about 5 microns is formed onsaid photoconductive material.

4. The method of claim 3 wherein said electrically conductive backingplate comprises predominantly aluminum.

5. An electrophotographic process wherein the reusable binder plate madeby the method of claim 1 is sequentially electrically charged, exposedto a light image pattern to be reproduced, and developed withelectrostatically attractable marking particles.

References Cited in the file of this patent UNITED STATES PATENTS2,158,111 Doolittle May 16, 1939 2,476,800 Blackburn July 19, 19492,494,920 Warrick Jan. 17, 1950 2,623,030 Cordier Dec. 23, 19522,663,636 Middleton Dec. 22, 1953 2,664,044 Dalton Dec. 29, 19532,693,416 Butterfield Nov. 2, 1954 2,803,542 Ullrich Aug. 20, 19572,808,328 Jacobs Oct. 1, 1957 2,829,025 Clemens et al. Apr. 1, 19582,860,048 Deubner Nov. 11, 1958 2,862,815 Sugarman et a1 Dec. 2, 19582,901,348 Dessauer et al. Aug. 25, 1959 2,979,402 Greig Apr. 11, 1961OTHER REFERENCES Wainer: Photographic Engineering, vol. 3, No. 1, pp.12-422 (1952).

UNITED STATES PATENT OFFICE, CERTIFICATE OF CORRECTION July 7, 1964Patent No, 3,140,174

I Harold E0 Clark It is hereby certified that error appears in the abovenumbered pattha,t the said Letters Patent should read as.

ent requiring correction and corrected below for the claim referencenumeral "1' Column 6, line 1,

read 3 d sealed this 221001 dayof December 1964,,

Signed an EDWARD J. BRENNER, Commissioner of Patents (SEAL) Attest:

ERNEST W. SWIDER Attesting Officer

1. A METHOD OF MAKING A SMOOTH SURFACE BINDER PLATE WHICH COMPRISES:DEPOSITING A LIGHT-SENSITIVE PHOTOCONDUCTIVE MATERIAL ON THE SURFACE OFAN ELECTRICALLY CONDUCTIVE BACKING PLATE TO FORM THEREBY APHOTOCONDUCTIVE LAYER HAVING AN IRREGULAR SURFACE, SAID PHOTOCONDUCTIVELAYER COMPRISING A MIXTURE OF PHOTOCONDUCTIVE MATERIAL AND A WATERINSOLUBLE RESINOUS BINDER, DEPOSITING OVER SAID IRREGULAR SURFACE ANAQUEOUS SOLUTION OF A WATER SOLUBLE ELECTRICALLY INSULATING MATERIAL,SAID SOLUTION USED BEING CHEMICALLY UNREACTIVE WITH SAID PHOTOCONDUCTIVEMATERIAL, ALLOWING SAID LIQUID CARRIER TO EVAPORATE FROM SAID SOLUTIONAND THE RESINOUS MATERIAL THEREIN TO THEREBY DEPOSIT IN AND FILL THEIRREGULARITIES OF SAID SURFACE WITH SAID RESINOUS MATERIAL, CONTINUINGSAID DEPOSITION UNTIL A SMOOTH RESINOUS FILM OF FROM ABOUT 0.1 TO ABOUT5 MICRONS IS FORMED ON SAID PHOTOCONDUCTIVE MATERIAL.